Could very well be over fill but I don't subscribe to the "run 1/2 quart low" theory. If the manufacturer calls for 4 quarts, the motor should run fine, without belching oil, with 4 quarts. Running 1/2 quart low may solve the oil belching problem but is eliminating a symptom and not necessarily addressing the issue.
The first couple of years the head breathers under the rocker supports were aluminum but were replaced with plastic units in '02 or maybe '03. Another bean counter bran fart decision from the MoCo; just not sure of the change year. The plastic units would warp from heat and lose their efficiency and, depending on mileage, the umbrella valves can harden and lose their seal as well as the breather mesh becoming clogged. In either case, sustained higher rpm running could "out run" the breathing capacity of the head breathers and result in oil puking out the air filter.
It is an easy thing to replace your breathers with the new stamped units (PN26500002). The later breathers are more efficient and have cured the oil puking problem for others. Just a suggestion as I don't believe that running a 1/2 quart low on oil is a good idea for an air cooled vtwin.:coffee